Peraluminious nepheline/kalsilite glass-ceramics

ABSTRACT

The present invention is concerned with glass-ceramic articles which are extremely resistant to impact and spontaneous delayed breakage, are capable of being sawn with a diamond wheel to a depth of over one-third the cross section thereof without breakage, and exhibit modulus of rupture values of at least 150,000 psi. The articles consist of a body portion and an integral surface compression layer having a depth of at least 0.005&#34;. The body portion consists essentially, in weight percent, of about 8-13% Na 2  O, 7-13% K 2  O, 30-36% Al 2  O 3 , 35-43% SiO 2 , and 6-10% RO 2 , wherein RO 2  consists of 6-10% TiO 2  and 0-4% ZrO 2 , wherein the molar ratio Al 2  O 3  :SiO 2  is &gt;0.5 but &lt;0.6 and the molar ratio K 2  O:Na 2  O is &gt;1:3 but &lt;1 and contains nepheline solid solution crystals corresponding to the formula Na 8-x  K x  Al 8  Si 8  O 32 , with x varying from 0.25-4.73, as the predominant crystal phase. The surface layer contains kalsilite as the predominant crystal phase and is produced by subjecting the base glass-ceramic article to an ion exchange reaction wherein K +  ions replace at least part of the Na +   ions in the nepheline solid solution crystals to convert those crystals to kalsilite.

BACKGROUND OF THE INVENTION

As is explained in U.S. Pat. No. 2,920,971, the basic patent in thefield of glass-ceramics, such products are prepared through the heattreatment of precursor articles. Thus, the three basic steps underlyingthe production of glass-ceramics comprise: (1) a glass forming batch,customarily containing a nucleating agent, is melted; (2) the melt issimultaneously cooled to a temperature below the transformation rangethereof and a glass article of a desired configuration shaped therefrom;and (3) the glass article is exposed to a predetermined heat treatmentschedule to cause the glass to crystallize in situ. Most frequently,this last step is broken into two parts. First, the parent glass articleis initially heated to a temperature in or slightly above thetransformation range to develop nuclei in the glass. Thereafter, theglass article is heated to a higher temperature, often above thesoftening point thereof, to cause the growth of crystals upon thepreviously-developed nuclei.

Because the mechanism of crystallization involves the essentiallysimultaneous growth of crystals upon a myriad of previously-developednuclei, the microstructure of a glass ceramic product consists ofrelatively uniformly-sized, fine-grained crystals homogeneouslydispersed within a glassy matrix, the crystals comprising thepredominant portion of the article. Glass-ceramic articles have beengenerally defined as being at least 50% crystalline and, in manyinstances, approach 100% crystallinity. This feature of highcrystallinity results in the physical properties exhibited byglass-ceramics being normally materially different from those of theprecursor glass and more nearly akin to those demonstrated by thecrystals.

The crystal phases developed in a glass-ceramic product are dependentupon the composition of the parent glass and the heat treatment appliedto the glass. The term "nepheline" has been utilized in the literatureto designate a natural mineral having a crystal structure categorized inthe hexagonal crystal system and indentified by the general chemicalformula (Na,K)ALSiO₄. It has been observed in Donnay et al., however,that the mineral nepheline exists in a wide range of solid solutions,the extent of which is not fully elucidated by the above formula (PaperNo. 1309 of the Geophysical Laboratory entitled "Nepheline SolidSolutions").

A similar situation exists in the glass-ceramic art where the range ofsolid solution is even more extensive because the growth of crystalstakes place under nonequilibrium conditions. Hence, metastable crystalglasses can be grown. Thus, the term "nepheline" is employed in theglass-ceramic art to indicate a wide range of solid solution crystalphases having general characteristics corresponding to those of themineral. Accordingly, whereas the crystals may vary substantially incomposition, they have a common diffraction peak pattern when studiedvia X-ray diffraction analysis. In sum, whereas any nepheline crystalwill display a characteristic pattern of diffraction peaks, it will beappreciated that the spacing and intensity of the peaks may varydepending upon the nature of the crystal phase.

Research has shown, however, that if potassium ions are substituted forsodium ions in the nepheline crystal, there is a tendency for thenepheline crystal phase to convert, in part at least, to a differenttype of crystal known as kalsilite. This potassium-containing crystal isalso classified in the hexagonal crystal system and, hence, likened tothe nepheline system but having a somewhat dissimilar crystal structure,as evidenced by a different pattern of diffraction peaks in an X-raydiffraction pattern analysis. This phenomenon is discussed in "TheNepheline-Kalsilite System:(I.) X-ray Data for the Crystalline Phases",J. V. Smith and O. F. Tuttle, American Journal of Science, 255, pp.282-305, April 1957.

U.S. Pat. No. 3,573,072 describes a method for chemically strengtheningglass-ceramic articles wherein the predominant crystal phase consistsessentially of nepheline solid solutions corresponding generally inchemical composition to the formula Na_(8-x) K_(x) Al₈ Si₈ O₃₂. In thatformula x may vary from about 0.25-4.73. The inventive method comprisedexposing such glass-ceramic bodies to an external source of K⁺, Rb⁺,and/or Cs⁺ ions at temperatures between about 400°-950° C. for asufficient length of time to effect replacement of Na⁺ ions in thenepheline crystals at the surface of bodies with K⁺, Rb⁺, and/or Cs⁺ions, and thereby convert said nepheline crystals into kalsilite and/orcrystals resembling synthetic kaliophylite. That crowding in of thelarger K⁺, Rb⁺, and/or Cs⁺ ions into the structure of the nephelinecrystals sets up compressive stresses in an integral surface layer onthe bodies, thereby imparting much improved mechanical strengthsthereto. Customarily, the external source of K⁺, Rb⁺, and/or Cs⁺ ionsconsisted of a bath of a molten salt containing K⁺, Rb⁺, and/or Cs⁺ions.

As is explained in that patent, the inventive article consisted of aglass-ceramic having an original nepheline solid solution crystal phasecontaining K³⁰ and Na⁺ ions, the ratio of the K⁺ -to-Na⁺ ions beinggreater than 0.25:1.75 on an ionic basis, the article beingdistinguished in having an integral surface layer in which at least aportion of the Na⁺ ions in the original nepheline phase is replaced bythe larger K⁺, Rb⁺, and/or Cs⁺ ions to develop a surface layer ofmodified chemical composition having a degree of compressive stressgenerated therein by the ion replacement. The exchange of Na⁺ ions withK⁺, and/or Cs⁺ ions takes place on a one-for-one basis such that theconcentration of the larger cations is greater in the surface layer thanin the interior portion of the article and the concentration of the Na⁺ions is greater in the interior portion than in the surface layer, thosedifferences in concentration causing the nepheline crystal structure toexpand and transform to a crystal phase with a larger unit cell volume,viz., kalsilite, thereby setting up compressive stresses.

The presence of nepheline and kalsilite solid solutions was detected bymeans of X-ray diffraction analyses, permitting at least a qualitativeestimate of the relative proportion of each in a particularglass-ceramic article. It was also observed that the various ions, andespecially the alkali metal ions, tended to appear in the crystal phasein essentially the same proportion as present in the parent glasscomposition. Finally, where a greater proportion of K⁺ ions is presentthan that indicated in the above formula, a different type of crystal,viz., kaliophylite, tends to develop as the original crystal phase inthe glass-ceramic.

The patent emphasized that the ratio of K⁺ ions-to-Na⁺ ions on an ionicbasis in the parent glass must be at least 0.25:1.75 and, preferably,greater than 1:4, in accordance with the above-cited ionic formulation.In the latter instance, an increase in mechanical strength, utilizing aK⁺ -for-Na⁺ ion exchange, can be achieved at temperatures between about400°-600° C. within 24 hours. Higher temperatures may be employed,however, to increase the rate of exchange, if desired. Modulus ofrupture values of 200,000 psi were observed when an exchange temperatureof 730° C. was utilized. The exchange of Rb³⁰ and/or Cs⁺ ions for Na⁺ions required higher temperatures, i.e., 750-950° C., to attain thedesired improved mechanical strengths. Glass compositions operable inthe patented invention consisted essentially, expressed in weightpercent on the oxide basis, of about 1-15% K₂ O, 5-20% Na₂ 0, 25-50% Al₂O₃, 25-50% SiO₂, and 5-15% TiO₂.

SUMMARY OF THE INVENTION

Although the improvement in mechanical strength resulting from themethod disclosed in U.S. Pat. No. 3,573,072 was readily reproducible,catastrophic failure was easily induced via notching the surface orthrough modest point impact. Moreover, the strengthened articles werefrequently the subject of spontaneous delayed breakage resulting fromdelayed flow propagation, presumably due to stress corrosion effectsfrom humid environments. It was postulated that thin surface compressionlayers were more vulnerable to those effects and in the patentedproducts the depth of the compression layer did not exceed about0.003"(˜75 microns) in the articles demonstrating very high strengths.Ion exchange treatments at temperatures above 703° C. were investigatedbut the strengths resulting therefrom were not very high, the causetherefor being conjectured as involving viscous relaxation effects atthose temperatures.

One significant feature observed via electron microscopy an X-raydiffraction in the glass-ceramic articles disclosed in U.S. Pat. No.3,573,072 is the substantial quantity of residual glass phase therein.Hence, even when the precursor glass composition was near the actualstoichiometry of nepheline, the nepheline solid solution crystallized insitu comprised only about two-thirds of the volume of theglass-ceramics. Thus, it seems evident that the composition of thenepheline solid solution crystals was substantially different from thatof the glass. Inasmuch as the literature indicates that the proportionsof Al₂ O₃ and SiO₂ do not vary appreciably in the nephelinecompositions, the alkali metal oxides Na₂ O and K₂ O are believed to bepartitioning differently in the crystal and in the glass. Microscopicstudy of the products determined that those compositions having higherlevels of K₂ O contained more glassy phase. Accordingly, it ishypothesized that the nepheline solid solution crystals are richer inthe NaAlSiO₄ nepheline component and the glass is proportionately richerin KALSiO₄.

The primary objective of the present invention is to develop, throughchemical strengthening, glass-ceramic articles containing nephelinesolid solution crystals corresponding generally in chemical compositionto the formula Na_(8-x) K_(x) Al₈ Si₈ O₃₂ having a much deeper surfacecompression layer than the products of U.S. Pat. No. 3,573,072 withoutloss of mechanical strength. It was hypothesized that this objectivemight be accomplished in two ways. First, removing or at least reducingthe amount of residual glassy phase in the glass-ceramic, this glassbeing the major source of stress relaxation via viscous flow. Second,significantly raising the strain point of the residual glass throughalteration of its composition in order to produce a more polymerizednetwork. Either of those accomplishments would permit maintenance ofhigh compressive stresses at temperatures where diffusion kinetics wouldallow the development of deep surface layers of kalsilite.

The first proposal, viz., to remove or reduce the amount of residualglass, was not deemed to be practical. Potassium aluminosilicate glassesare among the most thermally stable, i.e., they do not crystallizeeasily at temperatures below about 1200° C. Consequently, this inventionis directed toward modifying the composition of the residual glass suchthat it would exhibit a strain point at temperatures above 800° C.

We have discovered that the strain point of the residual glass can beraised to levels above 800° C. by increasing the molar ratio of Al₂ O₃to SiO₂ such that it is greater than 0.5 (the ratio of Al₂ O₃ :SiO₂ inclassic nepheline). Whereas any increase in Al₂ O₃ content will effect arise in the strain point of the residual glass, laboratory work hasindicated that a molar ratio Al₂ O₃ :SiO₂ of at least 0.51 is necessaryto demonstrate a dramatic effect. However, to insure the desiredextensive crystallization of nepheline solid solution, the molar ratioAl₂ O₃ :SiO₂ will not be permitted to exceed about 0.6.

When the molar ratio K₂ O:Na₂ O is raised above that existing in classicnepheline, i.e., about 1:3, the amount of residual glass present in theglass-ceramic article increases but, with the increased Al₂ O₃ content,the glass remains stiff, i.e., resistant to flow at temperatures of 850°C. and higher. Nevertheless, as the K₂ O content is raised, there is anincreasing tendency for clustering of the crystals into loosespherulites. Consequently, a molar ratio K₂ O:Na₂ O of 1 is deemed to bea practical maximum. Furthermore, at that K₂ O level the liquidustemperature becomes very high.

A study of the mechanism underlying the crystallization in situ of TiO₂-nucleated glass-ceramic articles containing nepheline solid solution asthe predominant crystal phase has elucidated the following sequence: (1)amorphous phase separation with isolation of titanium-rich islands; (2)separation of anatase (TiO₂) nuclei; (3) nucleation of mestasblecarnegieite crystals upon those nuclei; and (4) the transformation ofthe carnegieite crystals to nepheline solid solution crystals.Examination of photomicrographs has indicated that primary grain growthof carnegieite takes place rapidly within one hour at 850° C. withcrystal sizes less than 0.1 micron at impingement. The transformationinto nepheline solid solution crystals can occur within two-to-threehours at 850° C. and in less than one hour at 900° C. It appears thatvery little grain growth accompanies this transformation. Thatcircumstance differs greatly from the five-to-ten fold increase incrystal size demonstrated through the conversion of metastable β-quartzsolid solutions to β-spodumene solid solutions in thecommercially-marketed, low expansion glass-ceramics.

As is well-recognized in the glass-ceramic art, crystallization proceedsmore rapidly at higher temperatures. However, in the present nephelinesolid solution-containing compositions, crystallization at temperaturesabove 1050° C. appears to result in substantial secondary grain growth.For example, after an exposure of four hours at 1100° C., the crystalshad increased in size to 0.5 micron. Laboratory experience hasdemonstrated that greater mechanical strengths through chemicalstrengthening can be achieved in the instant glass-ceramic where thecrystal size is maintained very fine-grained. Accordingly,crystallization temperatures no greater than about 1050° C. will beutilized. To insure more uniformly-sized, fine-grained crystallization,a nucleation step at temperatures between about 700°-800° C. willcommonly be employed.

The cause of this improved mechanical strength is not fully understood.However, it is believed that diffusion of K⁺ ions from a bath of amolten potassium salt or other source of K⁺ ions is more sluggish in thenepheline crystal lattice than in the more open and less dense glassyphase or along grain boundaries. If, indeed, diffusion of K⁺ ions intothe nepheline crystal lattice is sluggish, reduced crystal sizes willpermit more efficient exchange of K⁺ -for-Na⁺ ions. Moreover, electronmicroscopy examination of the ion exchange boundary (the interfacebetween nepheline solid solution crystals and the kalsilite resultingfrom the chemical strengthening process) has evidenced at least asuggestion of zoning of kalsilite around individual nepheline solidsolution crystals. Where the crystals have diameters no greater thanabout 0.1 micron, such zoning would be of less practical significanceand greater utilization of the surface compression provided by the ionexchange reaction can be realized.

Both nepheline and kalsilite are stuffed derivations of the silicapolymorph tridymite in that each is derived from that structure byreplacing every second silicon ion with an aluminum ion and providingfor the necessary charge balance by stuffing sodium or potassium ionsinto the interstitial vacancies located along hexagonal channelsparallel to the c-axis. However, the structure of kalsilite is more openthan that of nepheline, i.e., the unit cell volume of kalsilite isgreater than that of nepheline, such that the transformation ofnepheline to kalsilite induced by the ion exchange reaction results inan increase of volume. For example, the change in volume for thetransformation of potassic nepheline (Na₃ KAl₄ Si₄ O₁₆) to kalsilite(KAlSiO₄) is about 8.2%, a very substantial increase for a displacive ornon-reconstructive type of transformation. This expansion, which resultsfrom the chemical strengthening ion exchange reaction of K⁺ ions for Na⁺ions, produces a highly expanded surface structure without any breakingof network Si-O or Al-O bonds. As a consequence, high compressive stresscan be generated in a surface layer with no weak or broken bondedinterface being formed during the transformation of nepheline tokalsilite or being "frozen in " upon cooling after termination of theion exchange reaction.

As was observed above, the chemically strengthened glass-ceramicproducts disclosed in U.S. Pat. No. 3,573,072 were frequently subject tospontaneous delayed breakage, this failing being the result of arelatively thin surface compression layer (˜3 mils). Attempts to developdeeper surface layers by conducting the ion exchange at highertemperatures led to significant losses in mechanical strength.

The present inventive method provides nepheline solidsolution-containing glass-ceramic articles with thick (>0.005" and,preferably >0.015") surface compression layers which demonstrate abradedmodulus of rupture values in excess of 150,000 psi and, preferablygreater than 200,000 psi. The articles are extremely resistant to impactand to spontaneous delayed breakage. For example, rods of the inventivematerials have been sawn with a diamond wheel to a depth of overone-third the cross section thereof and, in some instances, overone-half of their diameter and subsequently held for weeks under ambienthumid conditions without breaking. Other sawn rods were exposed in anautoclave for three days operating at 300° C. under saturated steam(1250 psig) with no breakage occuring.

Three factors undergird the operability of the instant invention:

First, the composition of the percursor glass body must be so designedthat, upon crystallization in situ to a glass-ceramic, the residualglass will exhibit a strain point in excess of 800° C.;

Second, the crystallization of the parent glass body will be conductedat temperatures no higher than about 1050° C., preferably 900°-1050° C.,so that the crystals developed will be no greater than about 0.5 micronin diameter; and

Third, an ion exchange reaction involving the replacement of Na⁺ ionswith K⁺ ions will be carried out at temperatures in excess of 800° C.,normally 800°-900° C. and, preferably, in the vicinity of 850° C.

(An explanation for relying upon "abraded strength" measurements toindicate the practical mechanical strength of chemically strengthenedglass-ceramics and a description of a laboratory method for determining"abraded strength" values are reported in U.S. Pat. No. 3,573,072.)

The base composition of the precursor glass will approximate thestoichiometry of nepheline but the molar ratio Al₂ O₃ :SiO₂ will begreater than 0.5, commonly 0.51-0.6, and the molar ratio K₂ O:Na₂ O willexceed 1:3 but will not be greater than 1. With those constraints,operable glass compositions consist essentially, as expressed in weightpercent on the oxide basis, of about 8-13% Na₂ O, 7-13% K₂ O, 30-36% Al₂O₃, 35-40% SiO₂, and 6-10% RO₂, wherein RO₂ consists of 6-10% TiO₂ and0-4% ZrO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Table I reports a number of glass batches, reported in terms of parts byweight on the oxide basis as calculated from the batch ingredients,illustrating the composition parameters of the instant invention.Inasmuch as the sum of the individual components totals or approximatelytotals 100, for all practical purposes the compositions may be deemed tohave been recorded in terms of weight percent. The actual batchingredients may comprise any materials, either the oxide or othercompound, which, when melted together, will be converted into thedesired oxide in the proper proportions.

The batches were compounded, dry ballmilled together to assist insecuring a homogeneous melt, and placed into platinum crucibles. Thecrucibles were introduced into a furnace operating at 1650° C. and thebatches melted for 16 hours with stirring. 0.25" diameter cane wasformed on an updraw and the remainder of the melts poured into steelmolds to produce slabs having the approximate dimensions of 6"×6"×0.5".The slabs were immediately transferred to an annealer operating at 700°C.

                  TABLE I                                                         ______________________________________                                              1      2      3    4    5    6    7    8    9                           ______________________________________                                        SiO.sub.2                                                                           38.0   38.0   41.0 37.0 39.0 37.0 37.0 38.0 37.0                        Al.sub.2 O.sub.3                                                                    34.0   34.0   31.0 35.0 34.0 35.0 34.0 34.0 35.0                        Na.sub.2 O                                                                          12.0   11.0   11.0 11.0 11.0 11.0 9.0  11.0 11.0                        K.sub.2 O                                                                           8.0    10.0   9.0  9.0  9.0  10.0 12.0 9.0  9.0                         TiO.sub.2                                                                           8.0    7.0    8.0  8.0  7.0  7.0  8.0  8.0  8.0                         As.sub.2 O.sub.5                                                                    0.5    0.5    0.5  0.5  0.5  0.5  0.5  0.5  0.5                         ______________________________________                                              10     11     12   13   14   15   16   17   18                          ______________________________________                                        SiO.sub.2                                                                           39.0   39.0   38.0 40.0 38.0 37.0 38.0 38.0 37.8                        Al.sub.2 O.sub.3                                                                    34.0   34.0   34.0 30.0 35.0 34.0 34.0 34.0 33.8                        Na.sub.2 O                                                                          11.0   12.0   12.0 11.0 11.0 10.0 10.0 9.0  11.9                        K.sub.2 O                                                                           9.0    8.0    9.0  8.0  9.0  11.0 11.0 12.0 8.0                         TiO.sub.2                                                                           7.0    7.0    7.0  7.0  7.0  8.0  7.0  7.0  8.0                         As.sub.2 O.sub.5                                                                    0.5    0.5    0.5  0.5  0.5  0.5  0.5  0.5  0.5                         ZrO.sub.2                                                                           --     --     --   --   --   --   --   --   --                          ______________________________________                                              19     20     21                                                        ______________________________________                                        SiO.sub.2                                                                           38.0   38.0   38.0                                                      Al.sub.2 O.sub.3                                                                    34.0   34.0   34.0                                                      Na.sub.2 O                                                                          11.0   11.0   12.0                                                      K.sub.2 O                                                                           10.0   10.0   8.0                                                       TiO.sub.2                                                                           6.0    5.0    7.0                                                       As.sub.2 O.sub.5                                                                    0.5    0.5    0.5                                                       ZrO.sub.2                                                                           2.0    3.0    2.0                                                       ______________________________________                                    

Although the above exemplary compositions represent laboratory melts, itwill be appreciated that the recited batches could be melted in largescale commercial units. TiO₂ is included as a nucleating agent and As₂O₅ performs its customary function as a fining agent.

Table II reports crystallization schedules applied to cane samplesand/or to rod samples of 0.25" cross section cut from annealed slabs ofthe above glasses. The general practice involved placing the samplesinto an electrically-fired furnace, heating the samples at about 300°C./hour to the first temperature dwell, further heating the samples atfurnace rate to the second temperature hold, and then cooling at furnacerates to 400°-500° C., opening the furnace door ajar, and cooling theroom temperature.

Table II also records the chemical strengthening parameters applied tothe samples. K⁺ ions were provided by a molten salt mixture of 52% byweight KCl and 48% K₂ SO₄. The salt was melted and held in 96% silicacontainers, marketed by Corning Glass Works, Corning, New York under thetrademark VYCOR. The molten salt attacks such metals as stainless steel.

Finally, Table II lists modulus of rupture measurements carried out inthe conventional manner on the cane and rod samples. In each instance,the samples were abraded before being tested.

                  TABLE II                                                        ______________________________________                                               Crystallization                                                                             Strengthening Modulus                                    Example                                                                              Treatment     Treatment     of Rupture                                 ______________________________________                                        1      720° C. for 4 hours                                                                  850° C. for 8 hours                                                                  202,000 psi                                       1025° C. for 4 hours                                            1      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  211,000 psi                                       1025° C. for 4 hours                                            2      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  207,000 psi                                       990° C. for 4 hours                                             2      760° C. for 4 hours                                                                  850° C. for 24 hours                                                                 182,000 psi                                       1025° C. for 4 hours                                            2      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  207,000 psi                                       1050° C. for 4 hours                                            2      800° C. for 4 hours                                                                  850° C. for 8 hours                                                                  217,000 psi                                       1025° C. for 4 hours                                            2      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  192,000 psi                                       990° C. for 4 hours                                             3      800° C. for 4 hours                                                                  850° C. for 8 hours                                                                  110,000 psi                                       990° C. for 4 hours                                             3      800°  C. for 4 hours                                                                 850° C. for 24 hours                                                                  74,000 psi                                       990° C. for 4 hours                                             4      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  199,000 psi                                       1025° C. for 4 hours                                            5      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  173,000 psi                                       990° C. for 4 hours                                             5      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  154,000 psi                                       1025° C. for 4 hours                                            5      760° C. for 4 hours                                                                  850° C. for 24 hours                                                                 155,000 psi                                       1025° C. for 4 hours                                            6      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  178,000 psi                                       990° C. for 4 hours                                             6      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  202,000 psi                                       1025° C. for 4 hours                                            6      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  210,000 psi                                       1050° C. for 4 hours                                            7      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  206,000 psi                                       950° C. for 4 hours                                             7      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  219,000 psi                                       1025° C. for 4 hours                                            8      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  186,000 psi                                       1025° C. for 4 hours                                            9      760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  172,000 psi                                       1025° C. for 4 hours                                            10     760° C. for 4 hours                                                                  800° C. for 8 hours                                                                  214,000 psi                                       1025° C. for 4 hours                                            10     760° C. for 4 hours                                                                  800° C. for 24 hours                                                                 187,000 psi                                       1025° C. for 4 hours                                            10     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  181,000 psi                                       1025° C. for 4 hours                                            10     760° C. for 4 hours                                                                  870° C. for 8 hours                                                                  160,000 psi                                       1025° C. for 4 hours                                            10     760° C. for 4 hours                                                                  890° C. for 8 hours                                                                  144,000 psi                                       1025° C. for 4 hours                                            11     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  194,000 psi                                       990° C. for 4 hours                                             11     760° C. for 4 hours                                                                  850° C. for 24 hours                                                                 177,000 psi                                       1025° C. for 4 hours                                            11     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  188,000 psi                                       1050° C. for 4 hours                                            12     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  174,000 psi                                       990° C. for 4 hours                                             12     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  165,000 psi                                       1025° C. for 4 hours                                            13     800° C. for 4 hours                                                                  850° C. for 8 hours                                                                  172,000 psi                                       1025° C. for 4 hours                                            13     720° C. for 4 hours                                                                  850° C. for 8 hours                                                                  152,000 psi                                       1025° C. for 4 hours                                            13     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  168,000 psi                                       990° C. for 4 hours                                             14     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  174,000 psi                                       990° C. for 4 hours                                             15     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  200,000 psi                                       1025° C. for 4 hours                                            16     760° C. for 4 hours                                                                  850°  C. for 8 hours                                                                 188,000 psi                                       950° C. for 4 hours                                             17     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  174,000 psi                                       1025° C. for 4 hours                                            18     760° C. for 4 hours                                                                  850° C. for 8 hours                                                                  210,000 psi                                       1025° C. for 4 hours                                            19     750° C. for 4 hours                                                                  850° C. for 8 hours                                                                  205,000 psi                                       1025° C. for 4 hours                                                   1050° C. for 4 hours                                            20     750° C. for 4 hours                                                                  850° C. for 8 hours                                                                  186,000 psi                                       1025° C. for 4 hours                                                   1050° C. for 4 hours                                            21     750° C. for 4 hours                                                                  850° C. for 8 hours                                                                  203,000 psi                                       1025° C. for 4 hours                                                   1050° C. for 4 hours                                            ______________________________________                                    

As noted above, the depth of the surface compression layer developed isa function of the time and temperature of the ion exchange treatment.The determination of an operable schedule to produce such a layer havinga depth of at least 0.005" is well within the technical ingenuity of theworker of ordinary skill in the art. In general, crystallization of theparent glass to a glass-ceramic will involve nucleation of about780°-800° C. for about 2-8 hours followed by crystallization at900°-1050° C. for about 4-24 hours.

That composition of the base precursor glass is critical and can be seenfrom an examination of Example 3. Likewise, the need for holding thetemperature for ion exchange below 900° C. is evidenced in Example 10.

We claim:
 1. A glass-ceramic article extremely resistant to impact andspontaneous delayed breakage, capable of being sawn with a diamond wheelto a depth of over one-third the cross section thereof without breakage,exhibiting a modulus of rupture of at least 150,000 psi, and wherein thecrystal present therein have diameters of less than about 0.5 micron,said article consisting of a body portion and an integral surfacecompression layer having a thickness of at least 0.005", said bodyportion consisting essentially, expressed in weight percent on the oxidebasis, of about 8-13% Na₂ O, 7-13% K₂ O, 30-36% Al₂ O₃, 35-43% SiO₂, and6-10% RO₂, wherein RO₂ consists of 6-10% TiO₂ and 0-4% ZrO₂, wherein themolar ratio Al₂ O₃ :SiO₂ is >0.5 but <0.6 and the molar ratio K₂ O:Na₂ Ois >1:3 but <1, and containing nepheline solid solution crystalscorresponding to the formula Na_(8-x) K_(x) Al₈ Si₈ O₃₂, with x varyingfrom 0.25-4.73, as the predominant crystal phase, and said surface layercontaining kalsilite as the predominant crystal phase.